GaN, AlN, or InN that is a nitride semiconductor, a material composed of a mixed crystal thereof, or the like has a higher saturated electron velocity or a wider band gap and is being studied for a higher withstanding voltage or higher output electronic device. For a higher withstanding voltage or higher output electronic device, a technique for a field-effect transistor (FET), in particular, a high electron mobility transistor (HEMT) is developed.
For an HEMT using a nitride semiconductor, a structure is provided in such a manner that an electron transit layer is formed of GaN and an electron supplying layer is formed of AlGaN. In the HEMT with such a structure, a higher concentration of 2-dimensional electron gas (2DEG) is produced by to a distortion caused by a difference between lattice constants of GaN and AlGaN, namely, a piezoelectric polarization, so that it is possible to obtain a semiconductor device with a higher efficiency and a higher output.
Meanwhile, a higher concentration of 2DEG is produced in the HEMT with a structure provided in such a manner that an electron transit layer is formed of GaN and an electron supplying layer is formed of AlGaN, so as to have a problem that it is difficult to attain normally-off-state. In order to solve such a problem, a method is disclosed for removing a part of an electron supplying layer in an area where a gate electrode is to be formed, so that a recess is formed to eliminate a 2DEG immediately below the gate electrode (for example, Japanese Patent Application Publication No. 2009-076845). Furthermore, a method is disclosed for forming a p-GaN layer between a gate electrode and an electron supplying layer so as to suppress production of 2DEG immediately below the gate electrode and thereby attain a normally-off-state (for example, Japanese Patent Application Publication No. 2007-019309).
However, in a method for forming a recess, damage caused by etching at a time when the recess is formed may apply to even the neighborhood of an electron transit layer, and thereby, a characteristic of an HEMT may be degraded by an increase of on-resistance, an increase of leakage current, or the like.
Furthermore, when a p-GaN layer is formed between an electron supplying layer and a gate electrode, the p-GaN layer is generally formed by forming a p-GaN film on an entire surface of the electron supplying layer and subsequently removing the p-GaN film in an area except an area where the gate electrode is to be formed, by dry etching. However, GaN is a material that may be very difficult to apply dry etching thereto, and further, it may be impossible to conduct etching with a higher selection ratio between AlGaN and GaN, so that it may be difficult to remove only a p-GaN layer. Accordingly, a part of a p-GaN layer may remain in an area where the p-GaN layer should be removed or a part of an electron supplying layer may be removed in an area where the p-GaN layer should be removed. In any of such cases, an on-resistance may be increased, so that a characteristic of an HEMT may be degraded. Furthermore, if a deviation of a thickness of an electron supplying layer or the like may be caused by an irregularity of etching in dry etching, an irregularity of a characteristic of an HEMT may also be caused so that a yield thereof may be lowered.
Therefore, a method for manufacturing a semiconductor device that may be capable of having removed only a p-type layer such as a p-GaN layer in an area other than an area where a gate electrode is to be formed, without removing an electron supplying layer, and a semiconductor device manufactured thereby are desired.